Ultrasound imaging is widely used in medical examination, and is used in various clinical fields. Fluid or gel is used as an acoustic coupling in medical ultrasound imaging, and fluids with microbubbles/microspheres are used as ultrasound contrast agents. To ensure proper contact between the transducer/ultrasound probe and the skin/tissue to be examined a contact gel or liquid is used. The ultrasound contact gel is used to avoid air pockets between the transducer/probe and tissue, and to facilitate a good acoustic coupling between the surface of the ultrasound transducer and the tissue.
In many neurosurgical departments ultrasound is used for imaging of tumours in brain surgery. The purpose of ultrasound imaging is to locate the tumour and anatomical structures, as well as to identify residual tumour during surgery. High image quality should be sustained throughout the whole operation in order to monitor the progress of tumour resection. However, the progress of surgery may also cause more noise and more inaccurate display of the brain anatomy in the ultrasound images. The term artefact is in medical imaging used to describe any part of the image that does not accurately represent the anatomy of the subject being investigated, and it is well known that ultrasound is prone to several different types of artefacts. When using ultrasound in brain tumour surgery, the presence of artefacts may interfere the surgeon's interpretation of the images.
Ultrasound imaging is used in surgery, e.g. in neurological surgery i.e. brain surgery and heart surgery, to make sure that the damaged tissue, tumour etc. is completely removed, and that unnecessary resection healthy tissue is avoided. When contact gels or fluids are used in surgery particular requirements apply. The ultrasound contact gel must be sterile, non-toxic and easy to remove after use.
During brain tumour surgery a resection cavity is filled with saline water before ultrasound imaging, to enable propagation of sound and to prevent air artefacts. The difference in attenuation between brain and isotonic saline may cause artefacts that degrade the ultrasound images, potentially affecting resection grades and safety. The acoustic waves travel through the cavity filled with saline water before reaching the biological tissue. The attenuation of acoustic waves in saline water is very low compared to the attenuation of acoustic waves in biological tissue. The attenuation coefficient α for water is 0.0022 while for e.g. brain it is reported measured by various groups to be within approximately 0.4-1.0 (Duck F A, In Physical properties of tissue, Academic Press, LTD). A major component of the attenuation of sound in brain tissue is caused by absorption, in which part of the acoustic energy is converted to heat. These effects cause the acoustic waves propagating in saline water to have higher amplitudes than acoustic waves propagating an equal distance in biological tissue.
The total attenuation is estimated by the equation:Attenutation [dB]=α[dB/(MHz*cm)]*1 [cm]*f[MHz]Wherein α is the attenuation coefficient                l is the medium length (or propagating distance)        f is the frequency of the transmitted ultrasound wave        
If we select a frequency of 8 MHz and a propagating distance of 10 cm and assume an attenuation coefficient of 0.8 for brain this will result in an attenuation of 0.18 dB for ultrasound propagating in water and an attenuation of 64 dB for waves propagating in brain. This difference in attenuation can generate noise in the ultrasound images e.g. when ultrasound is used intraoperatively in brain tumour surgery. The ultrasound waves transmitted through the water filled resection cavity will have a large amplitude when arriving at the cavity walls, due to the low attenuation of water. Thus, the sound waves being reflected from the cavity wall will also have relatively high amplitudes, see FIG. 1. Further, the sound waves propagating further into the tissue will have relatively high amplitudes. Compared to sound waves that have propagated entirely in brain tissue with a relative high attenuation coefficient, these transmitted and reflected waves will be less damped and thereby have significantly higher amplitudes. The fluctuations in intensity observed in the ultrasound images make it very difficult to interpret the images, see FIG. 1. The bright rim observed at the cavity wall may mask the presence of residual tumour, or the high intensity regions extending from the cavity wall may be interpreted as hyperechoic tumour when it actually is normal brain tissue.
The presence of the hyperechoic rim in ultrasound imaging of a resection cavity is described in several papers regarding the use of ultrasound in brain tumour surgery. This enhanced signal appearing below the fluid filled cavity in the ultrasound images is regarded as one of the major imaging artefacts encountered in peroperative ultrasound imaging. It is an objective of the present invention to reduce the enhancement artefact/brightness artefact/bright rim effect seen in ultrasound imaging of body cavities, thus substantially improving the usefulness of ultrasound imaging in e.g. resection monitoring. The brightness enhancement of tissue located beneath fluid filled spaces has been observed in ultrasound imaging of cysts, blood vessels or other fluid filled spaces. Because of this apparent enhancement of the reflected echo this frequently encountered image artefact is often referred to as brightness artefacts.
U.S. Pat. No. 6,302,848 concerns a medical ultrasound coupling media and lubricant, in gel or liquid form, comprised of polyethylene oxide (PEO), and aqueous solvent solutions. The inventive coupling media provides long-term biocompatibility (bio-inert, bio-erodible or biodegraded and excreted) in vivo with human tissue and body fluids. The ultrasound coupling and lubricating media is formulated and manufactured in such manner and form that renders the acoustic media sterile, non-cross-linked, pseudoplastic, and containing acceptably low levels of pyrogens.
EP 1 671 656 discloses an acoustic coupling agent comprising polyvinylpyrrolidone (PVP), which may be transported inside the body such as during surgery and with invasive procedures. It is disclosed that such coupling agent additionally comprises 1-99 weight % glycols, polyols and/or fats and esters thereof.
U.S. Pat. No. 5,625,137 discloses an ultrasound phantom for use with an ultrasound scanner where the phantom includes a low scatter tissue mimicking material. The material comprises an aqueous mixture of large organic water soluble molecules and an emulsion of fatty acid esters mixed with a hydroxy compound soluble in water.
CN 101695576, [abstract only] discloses a medical ultrasonic sterilization couplant, which is prepared from the following components in part by weight: sodium hydroxide 5-20, glycerol 150-450, ethylparaben 2-9, carbomer 10-50, chlorhexidine hydrochloride (CHX-HCL) 0.2-0.8, ethylene diamine tetraacetic acid (EDTA) 0.3-1.2, and distilled water 725-2,000. The couplant has a good acoustic matching and coupling function, avoids corroding and swelling ultrasonic probes, has disinfection and sterilization functions, avoids stimulating and sensitizing skin mucosae and can realize synchronous, quick and continuous disinfection and sterilization of the ultrasonic probes and the skin mucosae and effectively reduce potential intra-hospital cross infection risks caused by the contact of the ultrasonic probes (therapeutic heads) with human bodies in medical supersonic inspection and treatment processes. The couplant is applicable to trans-skin mucosa, transvaginal and transrectal ultrasonic diagnosis and treatment.
U.S. Pat. No. 4,542,745 reveals ultrasonic emulsion fluids which depending on the constitution provide emulsions with desired attenuation characteristics. The outer phase of the emulsion is a water and velocity enhancer mixture. Enhancers are suitable alcohols such as ethylene glycol, propylene glycol or glycerol. An oil phase such as a silicone fluid constitutes the suspended phase of the emulsion.
US 2005/074407 discloses an in vivo biocompatible and bio-excretable lubricant and ultrasound coupling fluid or gel comprising polyvinylpyrrolidone (PVP) and/or polyvinyl alcohol (PVA). The couplant fluid or gel comprises polyvinylpyrrolidone and/or polyvinyl alcohol solutions in water to which humectants such as alkylene glycols and/or polyalkylene glycols are added to achieve desired tactile and drying characteristics. Additionally, such fluids and gels may be prepared by addition of organic and inorganic cross-linkers.